In designing advanced compact actuators, there is a need for miniaturized devices and instruments that can apply substantial forces. Actuator requirements are becoming more stringent in terms of mass, dimensions, power and cost. Improvements in actuator robustness and reliability associated with power efficiency and compact packaging can lead to effective devices that are significantly more capable and reliable at a lower cost.
The development of high performance and efficient powertrains (actuator and transmission) can be necessary to meet the radical design requirements of demanding next generation robotic systems, particularly in space robotics where weight, efficiency and compact forms are decisive for the application functionality (e.g., space manipulator joints, powered bionics, humanoid manipulators, and space exercise devices for astronauts). Such robotic applications require a new breed of actuators that have compact, configurable hardware and inherent mechanical compatibility and adaptability to robotic manipulation.
Previously, actuators have been dominated by the Harmonic Drives, which offer compact mechanisms with high-speed reductions. During the past 30 years, the use of Harmonic Drive transmissions in conjunction with high performance electric motors has been the state of the art for actuated joints in robotics. Harmonic Drives are primarily useful to develop compact, high torque outputs actuators. These actuated joints can have little to no backlash and require only a one-time dry lubrication, making them ideal for a wide variety of applications. The principle of operation of Harmonic Drives is based on a unique type of transmission mechanism comprising three concentric components, denoted by the Wave Generator, Flexpline, and Circular Spline. The Wave Generator consists of a bearing that is press fitted within an elliptically shaped steel disk and inserted within the Flexpline. The Flexpline is a compliant thin-walled steel cup that conforms to the shape of the wave generator, and has teeth on its external diameter that mates with the Circular Spline. The Circular Spline consists of a rigid steel ring with teeth on the internal diameter and represents the output. Harmonic Drives are designed such that the Flexpline has two teeth less than the Circular Spline, so that when the Wave Plug rotates one revolution, the Circular Spline is shifted by two teeth yielding very high torque advantages. However research on Harmonic Drives has shown that it can exhibit large non-linear behavior under high dynamic loads due to its flexible gear component being in series inside the transmission. This elastic element creates a low stiffness medium inside the transmission, which deforms under load in a way similar to backlash. The elastic component also introduces instabilities under high gain feedback loops that further deteriorate the control system performance of the actuated joint. Additionally, Harmonic Drives are only transmission systems and require specialty motors to perform as actuators.
Several other actuator systems involving high performance brushless motors in combination with high gear-ratio planetary gearheads have been used in an attempt to reduce size, cost and manufacturing complexity. Overall, most of these actuators are still too large because the motor is connected serially to the planetary gearbox and not integrated within the gearbox. Some work has been performed to reduce the size of the assembly of the serially connected brushless DC motor and of the planetary gears. For example, a compact inner rotor slotless brushless DC motor serially connected to a single stage planetary gearheads has been developed. However, the resultant actuator can only exhibit low-reduction ratio at 1/5 and lacks the ability to generate high amount of torque that is often desired in robotics. Another example is a slotless type brushless DC motor that is integrated with a planetary gearhead to function as a robotic actuator. The system attempts to reduce the cogging of the brushless DC motor by optimizing the number of gear teeth integrated on the stator and tooth-to-pole ratio. Efficiencies of 80 to 85% have been realized for 90 W of power output, and the backlash ranged from 50 to 20 arc-min. A major limitation of these devices has been the inability to generate high amount of torque larger than 20 Nm that is often desired in robotics, and exhibited relative higher backlash compared to Harmonic Drives.